Phase shifting mask, manufacturing method thereof, and exposure method using such a phase shifting mask

ABSTRACT

A quartz substrate includes a first light transmitting portion and a second light transmitting portion which transmit exposure light. The first and second light transmitting portions are formed such that exposure light transmitted through respective light transmitting portions are 180° out of phase with each other. A semi-light shielding film is located between first and second light transmitting portion and formed in a part of first and second light transmitting portions. Also, semi-light shielding film has transmittance of at least 3% and not more than 30%.

This application is a continuation of application Ser. No. 08/644,755filed May 10, 1996, now U.S. Pat. No. 5,698,348, which is a division ofSer. No. 298,098, filed Aug. 30, 1994, now U.S. Pat. No. 5,536,602.

Phase Shifting Mask, Manufacturing Method Thereof, and Exposure MethodUsing Such a Phase Shifting Mask

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phase shifting mask and amanufacturing method thereof as well as an exposure method using such aphase shifting mask.

2. Description of the Background Art

In semiconductor integrated circuitry, so many developments have beenmade for high integration and miniaturization. Accordingly, the rapiddevelopment has been made for miniaturization of a circuit pattern whichis formed on a semiconductor substrate (hereinafter referred merely toas a wafer).

Among other things, photolithography has been recognized widely as thebasic technology for pattern formation, in which various developmentsand improvements have already been made. However, as miniaturization ofpatterns proceeds, an improvement of resolution of the pattern isstrongly required.

Photography is a technology for transferring a mask (original) patternon a photoresist applied on a wafer and patterning an underlying film tobe etched. When transferring the photoresist, the photoresist isdeveloped. During development, the photoresist of the type in which aportion exposed to light is removed is called a positive typephotoresist, while the type in which a portion not exposed to light isremoved is called a negative type. Now, a conventional exposure methodutilizing the photolithography technology will be described.

FIG. 45 is a schematic diagram of an optical system for illustrating aconventional exposure method. In this optical system, referring to FIG.45, a pattern on a mask is reduced and projected onto a photoresistplaced on the wafer. The optical system includes an illumination opticalsystem covering from a light source to a photomask pattern, and aprojection optical system covering from the photomask pattern to thewafer.

The illumination optical system includes a mercury lamp 111 serving as alight source, a reflection mirror 112, a light collecting lens 118, afly eye lenses 113, a diaphragm 114b, light collecting lenses 116a-116c,a blind diaphragm 115, and a reflection mirror 117. The projectionoptical system includes telephoto lenses 119a-119b and a diaphragm 125.

In exposure operation, a beam of light 111a emitted from mercury lamp111 is reflected from reflection mirror 112, so that only a g-line(wavelength:436 nm), for example, is reflected to become a beam of lighthaving a single wavelength. Beam of light 111a then enters each lens113a constituting fly eye lens 113, and after that, passes throughdiaphragm 114b.

Light passes through a light path 111b produced by one lens 113aconstituting the fly eye lens, and light passes through a light path111c produced by fly eye lens 113.

Beam of light 111a transmitted through diaphragm 114b then passesthrough light collecting lens 116a, blind diaphragm 115 and lightcollecting lens 116b, and is reflected from reflection mirror 117 at apredetermined angle.

Reflected from reflection mirror 117, beam of light 111a transmitsthrough light collecting lens 116c and irradiates uniformly the entiresurface of a photomask 720 in which a predetermined pattern is formed.Then, beam of light 111a is reduced by projection lenses 119a, 119b by apredetermined times for exposing a photoresist 121a placed on asemiconductor wafer 121.

Generally, a resolution limit R (nm) in photolithography using thedemagnification exposure method is represented as

    R=k.sub.1 ·λ/(NA)

where λ is a wavelength (nm) of the light used, NA is a numericalaperture of a lens, and k₁ is a constant depending on a resist process.

As can be seen from the above expression, a method conceivable toimprove resolution limit R, i.e., to obtain a fine pattern, is to reducek₁ and λ values and increase an NA value. In other words, it issufficient to reduce the constant depending on the resist process whileshortening the wavelength and increasing NA.

However, improvement of light source or the lens is technicallydifficult, and a depth of focus δ of the lens (δ=k₂ ·λ/(NA)²) might beshallower by proceeding shortening of the wavelength and increasing ofNA, thus causing deterioration of the resolution.

With this in view, studies for attempting miniaturization of patterns byimproving the photomask are under way. Lately, a phase shifting mask hasbeen focused as a photomask capable of improving a resolution of thepattern. Now, a structure and the principle of such a phase shiftingmask will be described in comparison with an ordinary photomask. Thebelow description will be directed to a phase shifting mask of aLevenson system and a halftone system.

FIGS. 46A-46C represent, respectively, a cross section of a mask when aconventional photomask is used, an electric field on the mask, and agraph showing the light intensity on a wafer. Referring to FIG. 46A, ametal mask pattern 503 is formed on a glass substrate 501 in theconventional photomask. The electric field on such a conventionalphotomask is pulse-modulated spatially by metal mask pattern 503 asshown in FIG. 46B.

However, as can be seen from FIG. 46C, as the pattern is miniaturized,exposure light transmitted through the photomask also enters anunexposed region (a region where the exposure light is intercepted bymetal mask pattern 503) due to diffraction of light. Therefore, light isdirected also to the unexposed region on the wafer, thus decreasingcontrast of light (a difference in light intensity between the exposedregion and the unexposed region on the wafer). Consequently, theresolution is decreased, making transfer of fine patterns difficult.

FIGS. 47A-47C represent, respectively, a cross section of a mask when aphase shifting mask of the Levenson system is used, an electric field onthe mask, and a graph showing light intensity on a wafer. Referring toFIG. 47A, the phase shifting mask includes an optical member called aphase shifter 505 provided on the conventional photomask.

More particularly, a chromium mask pattern 503 is formed on glasssubstrate 501 so that exposure regions and shaded regions are provided,and phase shifters 505 are provided every other exposure region. Phaseshifter 505 serves to convert a phase of the transmitted light by 180°.

Referring to FIG. 47B, since phase shifters 505 are provided every otherexposure region as described above, the electric field on the mask isstructured such that phases of the light transmitted through the phaseshifting mask are inverted alternately by 180°. Phases of light are thusreversed in adjacent exposure regions, so that beams of light arecanceled with each other due to interference of light in a portion wherereverse-phased beams of light are overlapped.

As a result, as shown in FIG. 47C, the light intensity is reduced at aboundary of adjacent exposure regions, so that a sufficient differencein light intensity between exposure regions and unexposure regions onthe wafer can be secured. This allows improvement of the resolution fortransferring fine patterns.

FIGS. 48A-48C represent, respectively, a cross section of a mask when aphase shifting mask of the halftone system is used, the electric fieldon the mask, and a graph showing light intensity on the wafer. Referringto FIG. 48A, an optical member called a phase shifter 506 is alsoprovided in the phase shifting mask of the halftone system as in theabove-described Levenson system.

Only difference is that optical member 506 is formed only on an opaquefilm 503 on glass substrate 501, so that a two-layered structure ofphase shifter 506 and opaque film 503 is provided. Phase shifter 506serves to convert the phase of the transmitted light by 180° asdescribed above, and opaque film 503 serves to decay the intensity ofthe exposure light without completely intercepting the exposure light.

Referring to FIG. 48B, since the two-layered structure of phase shifter506 and opaque film 503 is provided as described above, phases of lightare converted by 180° alternately in the electric field on the mask,while at the same time the intensity of one phase becomes smaller thanthe other. More particularly, phases of light are converted by 180° dueto transmission through phase shifter 506, and the intensity of light isdecayed due to transmission through opaque film 503 such that apredetermined thickness of the photoresist can remain after development.The phases of the light are reversed in adjacent exposure regions sothat beams of light are canceled with each other in the region wherereverse-phased beams of light are overlapped.

As can be seen from FIG. 48C, the intensity of light can be reduced atan edge of the exposure pattern because the phase is reversed at theedge. Consequently, the difference in the light intensity between theregion where the exposure light is transmitted through opaque film 503and the region the light is not transmitted through the film becomesgreater, whereby the resolution of the pattern image can be improved.

As described above, there are many types of phase shifting masksincluding the Levenson system, the halftone system and the like. Amongothers, a good resolution can be obtained according to the principles ofa so-called phase shifting mask of the Levenson system which had beeninvented by Marc Levenson, and that system is considered as the mostfavorable system from the standpoint of resolution.

Various techniques have been invented and proposed for manufacturingsuch a phase shifting mask, however, none has been used in practice.Among these proposals, one prior art technique which is consideredsuperior is a manufacturing technique by Marc Levenson which isdescribed in Marc D. Levenson et al., "Phase-Shifting Mask Strategies:Isolated Dark Lines" MICROLITHOGRAPHY WORLD, pp.6-12, March/April 1192.

Therefore, a structure and a manufacturing method of a phase shiftingmask of the Levenson system according to the above technique will bedescribed below as a conventional first phase shifting mask.

FIG. 49 is a cross sectional view schematically showing a structure of aconventional first phase shifting mask. Referring to FIG. 49, aconventional first phase shifting mask 720 includes a quartz substrate701 and a light shielding film 703.

Trenches are formed with a predetermined depth on a main surface ofquartz substrate 701. A region where the trench is not formed serves asa first light transmitting portion 701a, while a region where the trenchis formed serves as a second light transmitting portion 701b. Lightshielding film 703 is formed on quartz substrate 701 so as to cover asidewall portion of the trench and to expose predetermined regions offirst and second light transmitting portions 701a and 701b. Lightshielding film 703 has transmittance of not more than 1%, and athickness of about 1000 Å when chromium (Cr) is used as a material.

First and second light transmitting portions 701a and 701b arestructured such that phases of the exposure light transmitted throughrespective portions are converted by 180°. Since the phases of exposurelight transmitted through adjacent light transmitting portions are thusconverted by 180°, the resolution can be improved, as described above.

A bottom wall of the trench is substantially perpendicular to a sidewallthereof.

Now, a manufacturing method of the first phase shifting mask will bedescribed below.

FIGS. 50-57 are schematic cross sectional views showing in this order amanufacturing method of the conventional first phase shifting mask.First, referring to FIG. 50, a chromium film 705a is formed on a surface701a of quartz substrate 701. A resist film 707a is applied on chromiumfilm 705a. Resist 707a is then exposed with light and developed.

Referring to FIG. 51, a resist pattern 707 having a desired shape isformed through the above exposure and development. Using resist pattern707 as a mask, anisotropic etching is carried out. Resist pattern 707 isthen removed.

Referring to FIG. 52, a chromium film pattern 705 is thus formed whereina shifter pattern is transferred.

Referring to FIG. 53, using chromium film pattern 705 as a mask,anisotropic etching is carried out on quartz substrate 701, whereby atrench is formed on a surface 701a of quartz substrate 701 fortransferring the shifter pattern. Chromium film pattern 705 is thenremoved.

Referring to FIG. 54, first and second light transmitting portions 701aand 701b are thus formed in quartz substrate 701.

Referring to FIG. 55, a chromium film 703a is formed on the entiresurface wherein first and second light transmitting portions 701a and701b are formed. A resist film 709a is applied on chromium film 703a.Resist film 709a is then exposed with light and developed.

Referring to FIG. 56, a resist pattern 709 having a desired shape isformed through the above exposure and development. Using resist pattern709 as a mask, anisotropic etching is conducted to form a lightshielding film 703 which exposes desired regions of first and secondlight transmitting portions 701a and 701b. After that, resist pattern709 is removed, thereby completing the conventional phase shifting mask720 shown in FIG. 57.

In the above-described manufacturing method of the conventional phaseshifting mask, resist films 707a and 709a are not applied directly onquartz substrate 701. Accordingly, compare to a manufacturing method ofthe phase shifting mask in which the resist film is directly applied onthe substrate (Japanese Patent Laying-Open Nos. 4-355758 and 2-211450),this phase shifting mask includes an advantage that a defect such as isdescribed below will easily be repaired.

As one example of the method in which the resist film is directlyapplied on the substrate, a manufacturing method of the phase shiftingmask described in Japanese Patent Laying-Open No. 4-355758 will bedescribed.

FIGS. 58-61 are schematic cross sectional views showing in this orderthe manufacturing method of the phase shifting mask described in theabove document. First, referring to FIG. 58, after a phase shifting film803 is formed on the surface of a quartz layer 801, a light shieldingfilm 805 is formed on the surface of the film 803.

Referring to FIG. 59, after a resist pattern 807 is formed on lightshielding film 805, light shielding film 805 is patterned by etchingwith using resist pattern 807 as a mask. Resist pattern 807 is thenremoved.

Referring to FIG. 60, a photoresist 809 is directly applied on thesurface of the patterned light shielding film 805 and phase shiftingfilm 803. After patterning photoresist 809 into a desired shape, phaseshifting film 803 is etched with using resist pattern 809 as a mask.

Referring to FIG. 61, a trench 811 is formed in phase shifting film 803.After that, resist pattern 809 is removed, thereby completing the phaseshifting mask.

In the manufacturing method of the phase shifting mask disclosed in theabove document, a shifter pattern is formed in phase shifting film 803after light shielding film 805 is patterned, whereby resist film 809 isdirectly applied on the surface of phase shifting film (substrate) 803in the step shown in FIG. 60.

In a typical method for applying the resist film, a pinhole 809a whichpenetrates through resist film 809 is generated as shown in FIG. 62. Ifetching is carried out by using resist pattern 809 as a mask with suchpinhole 809a generated, a configuration shown in FIG. 63 will beobtained.

Referring to FIG. 63, an etchant enters pinhole 809a, so that phaseshifting film 803 located at the bottom of pinhole 809a is also removedby etching, thus generating a defect. If the phase shifting maskincluding such a defect is used for exposure of a wafer, the phase ofexposure light is converted in the defect portion in addition to desiredregions. Therefore, the resist film applied on the wafer cannot beexposed into a desired shape.

Therefore, when the resist film is applied directly on the substrate,the defect of the shifter is introduced directly into the substrate uponcarrying out etching or the like with using the resist pattern as amask.

On the contrary, in the manufacturing method proposed by Marc Levenson,as described above, the resist film is not formed directly on the quartzsubstrate. Thus, even if etching is carried out to the underlying layerby using the resist film wherein pinholes are generated as a mask,direct formation of the defect of the shifter in the substrate can beprevented.

More specifically, referring to FIG. 64, even if a pinhole 707a isgenerated in a resist pattern 707, since a chromium film 705 is lyingunder resist pattern 707, only chromium film 705 at the bottom ofpinhole 707a is removed by etching after etching is carried out to theunderlying layer by using resist pattern 707 as a mask.

As can be seen in the figure, a pinhole defect (clear defect) 705agenerated at the bottom of pinhole 707a is not in quartz substrate 701but in chromium film 705 lying thereon. Thus, pinhole defect 705a can beeasily repaired by filling up by deposition of a carbon-based film 705cin accordance with an FIB (Focussed Ion Beam) method, as shown in FIG.65.

In some cases, as can be seen from FIG. 64, a remaining defect (opaquedefect) 705b is generated at a portion where the chromium film shouldhave been removed by etching. However, since such remaining defect 705bis not the type of defect which is formed directly in quartz substrate701, the defect can be repaired easily by removing by blow (melt) ofirradiation of laser using a YAG laser, as shown in FIG. 65.

Thus, the conventional manufacturing method proposed by Marc Levensonhas the advantage that defects can easily be repaired because the defectof the shifter is not formed directly in the substrate.

Now, a structure of a phase shifting mask of the halftone system will bedescribed below as a conventional second phase shifting mask.

FIG. 66 is a cross sectional view schematically showing a structure of aconventional second phase shifting mask. As can be seen from FIG. 66, aconventional second phase shifting mask 920 includes a quartz substrate901 and a semi-light shielding film 903. A trench is formed with apredetermined depth in a main surface of quartz substrate 901.

A region where the trench is formed serves as a first light transmittingportion 901a, while a region where the trench is not formed serves as asecond light transmitting portion 901b. Semi-light shielding film 903 isformed on quartz substrate 901 so as to cover a sidewall portion of thetrench and to expose a predetermined region of first light transmittingportion 901a. Semi-light shielding film 903 serves to reduce theintensity of exposure light transmitting through semi-light shieldingfilm 903 to such an extent that a photoresist on the wafer is notphotosensitized by the exposure light or a predetermined thickness ofthe photoresist is left after development.

First and second light transmitting portions 901a and 901b arestructured such that phases of the exposure light transmitted throughrespective portions are out of phase by 180°. As the phases of theexposure light transmitted through adjacent light transmitting portionsare thus converted by 180°, the resolution can be improved as describedabove.

In the meantime, a bottom wall of the trench is substantiallyperpendicular to a sidewall thereof.

With using conventional first and second phase shifting masks, asdescribed above, a higher resolution can be obtained compared to anordinary photomask. However, there are some problems in the conventionalfirst and second phase shifting masks such as difficulty in obtaining adesired pattern shape because of I! increase in complexity of a circuitpattern and II! generation of a defect during formation of a shift mask.We will discuss these problems more in detail in the following.

I! Increase in Complexity of the Circuit Pattern

The recent circuit patterns of semiconductor integrated circuits havebeen made smaller and become complex in order to obtain thesemiconductor integrated circuits which has a large capacity andmultiple functions. For instance, in a DRAM (Dynamic Random AccessMemory), periodic patterns are provided densely (hereinafter referredmerely to as a dense pattern) in each memory cell region, while circuitpatterns having individual functions are isolated from each other(hereinafter referred merely to as an isolated pattern) in itsperipheral circuit region.

If a circuit pattern in which a dense pattern and an isolated patternare mingled is to be formed by the phase shifting mask of the Levensonsystem, a good resolution can be obtained in the dense pattern, while atthe same time, the resolution is not so good in the isolated pattern. Inthe meantime, if the phase shifting mask of the halftone system is used,then the resolution is better in the isolated pattern than in the densepattern.

FIGS. 67A and 67B illustrate the reason why the phase shifting mask ofthe Levenson system cannot be used to obtain the isolated pattern at agood resolution, wherein FIG. 67A shows a cross sectional view of thephase shifting mask of the Levenson system, and FIG. 67B shows the lightintensity on the wafer when exposure is conducted by using the phaseshifting mask shown in FIG. 67A.

Referring to FIGS. 67A and 67B, the isolated pattern exists away fromthe other circuit pattern. Thus, an exposure region 701b (hereinafterreferred to as an isolated exposure region) constituting the isolatedpattern is spaced apart by a considerable amount from an exposure regionconstituting the other circuit pattern. Accordingly, the exposure lightdirected onto the wafer after being transmitted through isolatedexposure region 701b will not overlap the exposure light having areverse phase which is transmitted through the other exposure region.This prevents the phase shifting mask effect of obtaining a goodresolution by canceling reverse-phased beams of light with each otherdue to diffraction of light.

On the contrary, in the dense pattern, reverse-phased beams of light areoverlapped in adjacent exposure regions, as shown in FIG. 47. Thiscontributes to improvement of the resolution due to canceling of beamsof light in the region where beams of light are overlapped.

Next, FIGS. 68A and 68B illustrate the reason why the phase shiftingmask of the halftone system cannot be used to achieve a good resolutionin the dense pattern, wherein FIG. 68A shows a cross sectional view ofthe phase shifting mask of the halftone system, and FIG. 68B shows thelight intensity on the wafer when exposure is conducted by using thephase shifting mask shown in FIG. 68A.

Referring to FIGS. 68A and 68B, since circuit patterns are dense in thedense pattern, a plurality of exposure regions 901a, 901a are disposedin proximity to each other. A portion P₃ is thus generated wherein beamsof light of the same phase are overlapped with each other in adjacentexposure regions 901a. When the beams of light of the same phase areoverlapped, the light intensity cannot be diminished at the edge (P₃portion) of the exposure pattern because of a small intensity of thereverse-phased light transmitted through exposure region 901b. In thiscase, the resolution cannot be improved due to an insufficientdifference of light intensity between the exposure region and theunexposure region.

On the contrary, in the isolated pattern, a portion where beams of lightof the same phase are overlapped is not generated, as shown in FIG. 48.This contributes to an improvement of resolution because a sufficientdifference in light intensity can be obtained at the edge of theexposure pattern due to overlap of the reverse-phased beams of light.

Therefore, in the pattern wherein the dense pattern and the isolatedpattern are mingled, either one pattern cannot be formed with a goodresolution even though the conventional first or second phase shiftingmask is used. This prevents formation of a desired pattern in such acomplex circuit pattern as having the dense pattern and the isolatedpattern.

II! Generation of a Defect During Formation of a Shifting Mask

In the conventional manufacturing method proposed by Marc Levenson inwhich a shifter pattern is formed in quartz substrate 701, anisotropicetching is carried out to quartz substrate 701. In this respect, thereis a problem of difficulty in obtaining a predetermined shape pattern inthis manufacturing method because of the reasons such as (1) adherenceof the light shielding film to the quartz substrate is degraded, (2)foreign objects are likely to remain on the quartz substrate, and (3)disadvantage due to remaining defects are easily generated.

(1) Adherence of the Light Shielding Film to Quartz Substrate

In the conventional manufacturing method of the phase shifting mask,anisotropic etching is conducted to quartz substrate 701 in the stepsshown in FIGS. 52 and 53. The sidewall of the trench which is formed bythe etching is thus substantially perpendicular to the bottom wall ofthe trench. In the step shown in FIG. 55, chromium film 703a is formedalso to cover the trench. However, it is difficult to form the filmappropriately on the sidewall portion of the trench having theabove-described structure. Specially, if chromium 703a is formed by amethod having a poor step coverage such as sputtering, the appropriateformation of the film becomes more difficult.

This leads to deterioration of adherence of chromium film 703a againstquartz substrate 701 on the sidewall portion of the trench (a regionindicated by D₃ in the figure). When the adherence of chromium film 703ais not favorable, chromium film 703a will be peeled off easily duringthe cleaning step of the manufacturing process of the phase shiftingmask. Also, light shielding film 703 will be peeled off easily duringcleaning after completion of the phase shifting mask.

A portion where light shielding film 703 is thus peeled off becomes aso-called clear defect. When exposure is carried out onto the wafer byusing the phase shifting mask having the clear defect, a region whichshould not be exposed will be exposed with light on the wafer, thuspreventing formation of a pattern having a desired shape.

(2) Remaining of Foreign Objects on the Quartz Substrate

As described above, according to the manufacturing method of theconventional phase shifting mask, quartz substrate 701 is subjected toanisotropic etching in the step shown in FIGS. 52 and 53, so that thebottom wall of the trench formed during the etching is substantiallyperpendicular to the sidewall of the trench, as shown in FIG. 70. Sincean edge of the stepped portion (a region indicated by D₄ in the figure)is substantially 90°, a foreign object 750a is easily be trapped at theedge.

More specifically, after formation of the trench on the surface ofquartz substrate 701 in the steps shown in FIGS. 53 and 54, a chromiumfilm pattern 705 is removed. During this removal, a foreign object istrapped at the edge of the stepped portion. Also, foreign objects suchas being generated internally from the etching apparatus or included inan etching solution can be trapped.

If a light shielding film is formed with foreign objects being trapped,an etchant spreads under a resist 709 through foreign object (or aftermelting the foreign object at a high speed) during patterning of lightshielding film 703, as shown in FIGS. 71 and 72. Thus, light shieldingfilm 703 is removed excessively by etching as shown in FIG. 73. Thisexcessively-removed portion by etching becomes a so-called clear defect.A region which should not be exposed will be exposed with light on thewafer due to the clear defect, thus preventing formation of a patternhaving a desired shape.

As can be seen from FIG. 73, a portion of light shielding film 703indicated by an arrow K is not in contact with the sidewall of thestepped portion but is protruding into the space. This portion K oflight shielding film 703 is easily be peeled off by cleaning afterremoval of the resist or by cleaning after repairing of the defect oflight shielding film 703, thus generating the clear defect as in theabove.

In the meantime, as shown in FIG. 74, the surface of light shieldingfilm 703 which is formed on the edge of the stepped portion reflects thestepped shape of the underlying layer. Thus, a foreign object 750b willeasily be trapped at a portion along the edge of the stepped portion oflight shielding film 703 (a region indicated by D₅ in the figure), as inthe above.

If the trapped foreign object 750b is large, foreign object 750b willremain penetrating onto the light shielding portion in some cases. Ifforeign object 750b is made of a material through which the exposurelight cannot be transmitted, the foreign object will necessarily be aso-called opaque defect. Even if the exposure light can transmitsthrough foreign object 750b, a proper function of light shielding mask703 as a phase shifting mask will be prevented when a phase of thetransmitted light is shifted by a considerable amount (usually 10°-20°)or more by the material. Thus, the shape to be transferred onto theresist on the wafer is deformed, thus generating a defect.

Thus, when the foreign object is trapped in the stepped portion or thelike in quartz substrate 701 during processing of the phase shiftingmask, it is difficult to expose the resist on the wafer into a desiredshape.

(3) Disadvantage Caused By Remaining Defects

In some cases, during patterning of chromium film 705a shown in FIGS. 50and 51, a remaining defect 705b is generated as shown in FIG. 75. Such aremaining defect 705b can be repaired by the above-described laser blowor the like. However, this does not mean that all of generated remainingdefects 705b cannot be sensed and repaired. Also, it is sometimesdesired to omit a step of repairing for the sake of simplifying of amanufacturing process. In such a case, some remaining defects 705b arestill left.

In the manufacturing method of the conventional phase shifting mask, thetrench is formed on the surface of quartz substrate 701 by anisotropicetching in the steps shown in FIGS. 52 and 53. However, if anisotropicetching is carried out with remaining defect 705b being generated, anunetched region (a region indicated by D₆ in the figure) which shouldhave been etched is generated as shown in FIG. 76.

Referring to FIG. 77, the phases of the beams of exposure lighttransmitted through adjacent light transmitting portions are notconverted by 180° in the thus formed phase shifting mask. Moreparticularly, the exposure light transmitted through light transmittingportion 701a and the exposure light transmitted through a lighttransmitting portion 701ab have the same phase. Accordingly, theexposure light transmitted through both transmitting portions 701a and701ab will be intensified in a portion where those beams of light areoverlapped. Consequently, a difference of light intensity between theexposed region and the light-shielded region on the wafer becomes small,so that the resolution is degraded, and formation of a desired patterncannot be achieved.

Additionally, as shown in FIG. 78, if region 701ab which is not etcheddue to the remaining defect is generated partially at the lighttransmitting portion, the shape of transfer pattern will be deformed.

More particularly, phases of exposure light transmitted through unetchedregion 70lab and a region 701bb which is removed by etching are reverse.Accordingly, at an interface P between unetched region 701ab and etchedregion 701bb, a portion having the light intensity of zero is generateddue to canceling of exposure light.

The resist on the wafer cannot be exposed with light at such portionhaving the light intensity of zero. In other words, a region whereexposure should have been carried out is generated on the resist, sothat a resist pattern having a desired shape cannot be obtained. Ifpatterning of the underlying layer is carried out by using such a resistpattern, an insufficient pattern is formed.

Specially, when a negative type resist in which a portion shielded fromlight is removed by a developing solution is used, resist will not beleft at a portion having the light intensity of zero (region P). Thatis, a region wherein resist should have been left is generated. When aninterconnection layer, for example, is formed by patterning with usingsuch a resist pattern, the interconnection will be cut off at region Phaving the light intensity of zero, as shown in FIG. 79.

Thus, in addition to deterioration of the resolution, the pattern shapewill be deformed when the remaining defects are generated.

SUMMARY OF THE INVENTION

The present invention is made in view of the description in the aboveI!, II!, and an object of the present invention is to provide a phaseshifting mask which facilitates formation of a desired pattern shape, amanufacturing method of such a phase shifting mask, and an exposuremethod using such a phase shifting mask.

A phase shifting mask according to one aspect of the present inventionincludes a substrate and a semi-light shielding film. The substrateincludes a first and a second light transmitting portion fortransmitting exposure light. The second light transmitting portion isadjacent to the first light transmitting portion and transmits theexposure light to have a phase different from that of the exposure lighttransmitted through the first light transmitting portion. The semi-lightshielding film is located at an interface between adjacent first andsecond light transmitting portions, and is formed at a region of thefirst and the second light transmitting portion. The first lighttransmitting portion includes a first transmitting region and a firstattenuated transmitting region wherein a semi-light shielding film isformed. A light intensity of the exposure light transmitted through thefirst transmitting region is greater than that of exposure lighttransmitted through the first attenuated transmitting region. The secondlight transmitting portion includes a second transmitting region and asecond attenuated transmitting region wherein a semi-light shieldingfilm is formed. A light intensity of the exposure light transmittedthrough the second transmitting region is greater than that of theexposure light transmitted through the second attenuated transmittingregion. Transmittance of the semi-light shielding film is at least 3%and not more than 30%.

In the phase shifting mask according to one aspect of the presentinvention, when a distance between the first and the second transmittingregion spaced apart by the semi-light shielding film becomes close anddense (that is, a dense pattern), beams of exposure light transmittedthrough the first and the second transmitting region are overlapped atan edge portion of an exposure pattern. Since these overlapping beams ofexposure light are in different phases, that is, the phases are reversedfrom each other, the beams of exposure light are canceled with eachother in the overlapping portion. Thus, a portion having light intensityof zero is necessarily generated at the edge portion of the exposurepattern, so that a shape of the exposure pattern becomes sharp, thusimproving a resolution.

In the meantime, the semi-light shielding film has transmittance of atleast 3% and not more than 30% so that exposure light can be transmittedto a certain extent. In other words, the exposure light is alsotransmitted through the first and second attenuated transmitting region.Thus, even if a distance between the first and the second transmittingregions spaced apart by the semi-light shielding film becomes large sothat either one of the transmitting regions (for example, the firsttransmitting region) is isolated to be an isolated pattern, the exposurelight transmitted through the first transmitting region and the exposurelight transmitted through the second attenuated transmitting region areoverlapped at the edge portion of the exposure pattern. Since phases ofthese overlapping beams of exposure light are different, that is, thephases are reversed with each other, the beams of exposure light arecanceled with each other in the overlapping portion. Consequently, theportion having the light intensity of zero is necessarily be generatedat the edge portion of exposure pattern, so that the shape of theexposure pattern becomes sharp, thus achieving improvement of theresolution.

As described above, since a good resolution can be obtained in both thedense pattern and the isolated pattern, a desired pattern shape caneasily be formed even if a circuit pattern becomes complex.

It is noted that the intensity of light transmitted through thesemi-light shielding film must be adjusted such that a photoresist isnot photosensitized, or even after photosensitizing, a predeterminedthickness of the photoresist is left. When transmittance of thesemi-light shielding film exceeds 30%, the photoresist might bephotosensitized by the light transmitted through the semi-lightshielding film. Therefore, transmittance of the light semi-lightshielding film has to be not more than 30%.

In the meantime, if the intensity of light transmitted through thesemi-light shielding film is too weak, the effect of obtaining a sharpexposure pattern by virtue of overlapping of light which is out of phasewith the light transmitted through the semi-light shielding film cannotbe achieved. When transmittance of the semi-light shielding film becomesless than 3%, the above effect cannot be achieved because the lighttransmitted through the semi-light shielding film is too weak.Therefore, transmittance of the semi-light shielding film has to be notless than 3%.

In a phase shifting mask according to a preferred aspect of the presentinvention, a substrate includes a stepped portion of a predeterminedheight which is constructed by a surface of a first light transmittingportion and a surface of a second light transmitting portion. Thestepped portion of the substrate is covered by a semi-light shieldingfilm. A sidewall of the stepped portion of the substrate is in a shapehaving substantially the same radius of curvature as the height of thestepped portion.

In a phase shifting mask according to one preferred aspect of thepresent invention, the sidewall of the stepped portion of the substrateis in the shape having substantially the same radius of curvature as theheight of the stepped portion. In other words, a slope of the sidewallof the stepped portion is gentle. Therefore, even when the semi-lightshielding film is formed on the stepped portion, a favorable adherenceof the semi-light shielding film to the substrate can be obtained.

Also, since the sidewall of the stepped portion is gentle, trapping of aforeign object at a bottom of the stepped portion during cleaning can beprevented.

Therefore, a defect is unlikely to generate in the phase shifting mask,thus facilitating formation of a desired pattern shape.

In a phase shifting mask according to another preferred aspect of thepresent invention, a substrate includes a first film, and a second filmwhich is formed on the first film and made of a material having an underetching characteristic different from that of the first film, wherein asurface of a first light transmitting portion is formed by a surface ofthe first film, and a surface of a second light transmitting portion isformed by a surface of the second film.

In a phase shifting mask according to another preferred aspect of thepresent invention, the substrate includes first and second films whichare respectively made by materials having different under etchingcharacteristics. Thus, during etching of the second film for forming astepped portion on the substrate surface, the first film serves as anetching stopper layer. Accordingly, controllability of the height of thestepped portion formed on the substrate surface becomes favorable, thusfacilitating formation of the stepped portion having the predeterminedheight. Therefore, controllability of a phase shifting angle of thefirst and the second light transmitting portion can be very muchimproved, so that a desired pattern shape can be easily formed.

A phase shifting mask according to another aspect of the presentinvention includes a substrate and a light shielding film. The substrateincludes a first light transmitting portion through which exposure lightis transmitted, and a second light transmitting portion. The secondlight transmitting portion serves to convert a phase of exposure lighttransmitted therethrough into a reverse phase with respect to a phase ofexposure light which is transmitted through the first light transmittingportion. In the meantime, a stepped portion having a predeterminedheight is provided by a surface of the first light transmitting portionand a surface of the second light transmitting portion. The lightshielding film is provided to cover such a stepped portion of thesubstrate for intercepting exposure light such that predeterminedregions of the first and the second light transmitting portion areexposed. A sidewall of the stepped portion of the substrate is at itsbottom portion in a shape having substantially the same radius ofcurvature as the height of the stepped portion.

In the phase shifting mask according to another aspect of the presentinvention, the sidewall of the stepped portion of the substrate is inthe shape having substantially the same radius of curvature as height ofthe stepped portion. In other words, a slope of the sidewall of thestepped portion is gentle. Thus, even when the light shielding film isformed on the stepped portion, adherence of the light shielding film tothe substrate is favorable.

Also, since the sidewall of the stepped portion is gentle, trapping of aforeign object generating during cleaning at the bottom of the steppedportion can be prevented.

Therefore, a defect is unlikely to generate in the phase shifting mask,thus facilitating formation of a desired pattern shape.

In a phase shifting mask according to one preferred aspect of thepresent invention, a substrate includes a first film, and a second filmformed on the first film and made of a material having an under etchingcharacteristic different from that of the first film, wherein a surfaceof a first light transmitting portion is formed by a surface of a firstfilm and a surface of a second light transmitting portion is formed by asecond film.

In a phase shifting mask according to one preferred aspect of thepresent invention, the substrate includes first and second films whichare respectively made of materials having different under etchingcharacteristics. Thus, during etching of the second film for forming astepped portion on the substrate surface, the first film serves as anetching stopper layer. Accordingly, controllability of the steppedportion formed on the substrate surface is improved, thus facilitatingformation of the stepped portion having a predetermined height.Therefore, controllability of a phase shifting angle of the first andsecond light transmitting portions is very much improved, so that apredetermined pattern shape can easily be formed.

An exposure method using a phase shifting mask according to one aspectof the present invention includes the following steps.

Exposure light is emitted from a light source and directed to a phaseshifting mask. The exposure light is then transmitted through the phaseshifting mask and is projected onto a photoresist placed on a film to beetched, so that a predetermined region of the photoresist isphotosensitized. The phase shifting mask includes a substrate and asemi-light shielding film. The substrate includes a first lighttransmitting portion for transmitting exposure light and a second lighttransmitting portion. The second light transmitting portion is providedadjacent to the first light transmitting portion, and transmits theexposure light to have a phase different from that of the exposure lighttransmitted through the first light transmitting portion. The semi-lightshielding film is disposed at an interface between adjacent first andsecond light transmitting portions, and is formed in a region of thefirst and the second light transmitting portion. The first lighttransmitting portion includes a first transmitting region and a firstattenuated transmitting region wherein the semi-light shielding film isformed. Intensity of exposure light transmitted through the firsttransmitting region is greater than that of the exposure lighttransmitted through the first attenuated transmitting region. The secondlight transmitting portion includes a second transmitting region and asecond attenuated transmitting region wherein the semi-light shieldingfilm is formed. Intensity of the exposure light transmitted through thesecond transmitting region is greater than that of the exposure lighttransmitted through the second attenuated transmitting region.Transmittance of the light shielding film is at least 3% and not morethan 30%.

In the exposure method using the phase shifting mask according to oneaspect of the present invention, the phase shifting mask capable ofachieving a good resolution in both a dense pattern and an isolatedpattern is used. Formation of a desired pattern shape can easily beachieved even in a circuit pattern wherein the dense pattern and theisolated pattern are mingled.

Also, since both the dense pattern and the isolated pattern can beformed by the phase shifting mask, it is possible to set coherency σ tobe an appropriate value. Thus, a phase shifting effect becomes moreconspicuous, so that the resolution is improved and a desired patternshape can easily be formed.

An exposure method using a phase shifting mask according to anotheraspect of the present invention includes the following steps.

First, exposure light is emitted from a light source and is directed toa phase shifting mask. The exposure light transmitted through the phaseshifting mask is then projected onto a photoresist placed on a film tobe etched, so that a desired region of the photoresist isphotosensitized. The phase shifting mask includes a substrate and alight shielding film. The substrate includes a first light transmittingportion for transmitting exposure light, and a second light transmittingportion. The second light transmitting portion transmits the exposurelight in a phase different from that of the exposure light transmittingthrough the first light transmitting portion. A stepped portion having apredetermined height is formed by surfaces of the first and second lighttransmitting portions. The light shielding film is provided to cover thestepped portion of the substrate and to expose predetermined regions ofthe first and the second light transmitting portion. Also, a sidewall ofthe stepped portion of the substrate is in a shape having substantiallythe same radius of curvature as the height of the stepped portion.

In the exposure method using the phase shifting mask according toanother aspect of the present invention, the phase shifting mask inwhich adherence of the light shielding film to the substrate isfavorable and trapping of a foreign object is unlikely to occur is used.Therefore, an unsatisfactory pattern shape due to defects is unlikely togenerate, so that a desired pattern shape can easily be formed.

A manufacturing method of a phase shifting mask according to one aspectof the present invention includes the followings steps.

First, a substrate is formed which includes a first light transmittingportion for transmitting exposure light, and a second light transmittingportion which is provided adjacent to the first light transmittingportion, and transmits the exposure light to have a phase different fromthat of the exposure light transmitted through the first lighttransmitting portion. Then, a semi-light shielding film is formed on thesubstrate so as to be located at an interface between adjacent first andsecond light transmitting portions. The first light transmitting portionincludes a first transmitting region and a first attenuated transmittingregion wherein the semi-light shielding film is formed, intensity of theexposure light transmitted through the first transmitting region beinggreater than that of the exposure light transmitted through the firstattenuated transmitting region, and the second light transmittingportion includes a second transmitting region and a second attenuatedtransmitting region wherein a semi-light shielding film is formed,intensity of the exposure light transmitted through the secondtransmitting region being greater than that of the exposure lighttransmitted through the second attenuated transmitting region.Transmittance of the semi-light shielding film is set at least 3% andnot more than 30%.

In the manufacturing method of the phase shifting mask according to oneaspect of the present invention, a good resolution can be obtained inboth a dense pattern and an isolated pattern, whereby the phase shiftingmask capable of easily achieving a desired pattern shape can bemanufactured.

A manufacturing method of a phase shifting mask according to anotheraspect of the present invention includes the following steps.

First, a mask having a predetermined shape is formed on a main surfaceof a substrate which transmits exposure light. By isotropically etchingthe main surface of the substrate using a mask, a first lighttransmitting portion, and a second light transmitting portion whichtransmits the exposure light to have a phase different from the exposurelight transmitted through the first light transmitting portion areformed on the substrate. Then, a light shielding film which interceptsthe exposure light is formed on the main surface of the etched substrateso as to expose predetermined regions of the first and the second lighttransmitting portion.

In the manufacturing method of the phase shifting mask according to thepresent invention, the main surface of the substrate is isotropicallyetched. Thus, a sidewall of the stepped portion formed through theetching has a gentle slope compared to the case when anisotropic etchingis carried out. Accordingly, even when the light shielding film isformed on the stepped portion, adherence of the light shielding film tothe substrate is still favorable.

Also, trapping of a foreign object generated during cleaning at thebottom of the stepped portion can be prevented by virtue of a gentlesidewall of the stepped portion.

In the meanwhile, during anisotropic etching, an etchant spreads under amask or the like. Therefore, even if a remaining defect is generated onthe substrate, the etchant spreads under the remaining defect duringetching to remove that portion of the substrate. The remaining defectthen falls from the substrate because the underlying layer is lost. Inthis respect, even if the remaining defect exists, a portion which isleft without being etched will not be generated in a region to beremoved by etching on the substrate after isotropic etching of thesubstrate. When the resist film placed on the wafer is exposed by usingthe thus formed phase shifting mask, degradation of the resolution anddeformation of the pattern shape are prevented.

As described above, defects are unlikely to generate in the phaseshifting mask. Therefore, the phase shifting mask in which a desiredpattern shape can easily be formed is manufactured.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically showing a structure of aphase shifting mask according to a first embodiment of the presentinvention.

FIG. 2 is a schematic cross sectional view showing a manufacturingmethod of the phase shifting mask according to the first embodiment ofthe present invention.

FIG. 3A is a schematic cross sectional view of the phase shifting maskof the present invention for illustrating that a good resolution can beobtained in a dense pattern in the phase shifting mask according to thefirst embodiment of the present invention.

FIG. 3B is a graph showing light intensity on a wafer when the phaseshifting mask shown in FIG. 3A is used.

FIG. 4A is a schematic cross sectional view of the phase shifting maskof the present invention for illustrating that a good resolution can beobtained in an isolated pattern in the phase shifting mask according tothe first embodiment of the present invention.

FIG. 4B is a graph showing light intensity on a wafer when the phaseshifting mask shown in FIG. 4A is used.

FIG. 5 is a graph showing a relationship between exposure amount forexposing a resist and thickness of a resist film left after development.

FIG. 6A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the first embodiment of the presentinvention when used to a pattern in which the dense pattern and theisolated pattern are mingled.

FIG. 6B is a cross sectional view schematically showing a resist patternformed by the phase shifting mask shown in FIG. 6A.

FIG. 7A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the first embodiment of the presentinvention when used to a pattern in which the dense pattern and theisolated pattern are mingled.

FIG. 7B is a cross sectional view schematically showing a resist patternformed by the phase shifting mask shown in FIG. 7A.

FIG. 8 is a graph showing a relationship between size of a lightshielding film on a wafer and the depth of focus plotted for a phaseshifting mask of the Levenson system and an ordinary mask.

FIG. 9A is a cross sectional view schematically showing an ordinaryphase shifting mask of the Levenson system for illustrating arelationship between size of the light shielding film and an exposurepattern.

FIG. 9B is a graph showing light intensity when the size of the lightshielding portion is greater than 2λxn.

FIG. 9C is a graph showing light intensity when the size of the lightintensity when the size of the light shielding portion is smaller than2λxn.

FIG. 10 is a schematic diagram of an optical system for implementing anexposure method using a phase shifting mask according to the firstembodiment of the present invention.

FIG. 11 is a plan view showing a specific circuit pattern including adense pattern and an isolated pattern.

FIG. 12 is a plan view schematically showing a structure of a diaphragm.

FIG. 13 is a cross sectional view schematically showing a structure of aphase shifting mask according to a second embodiment of the presentinvention.

FIGS. 14-16 are cross sectional views schematically showing in thisorder a manufacturing method of the phase shifting mask according to thesecond embodiment of the present invention.

FIG. 17A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the second embodiment of thepresent invention when used to a pattern in which the dense pattern andthe isolated pattern are mingled.

FIG. 17B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 17A.

FIG. 18 is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the second embodiment of thepresent invention when used to a pattern in which the dense pattern andthe isolated pattern are mingled.

FIG. 18B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 18A.

FIG. 19 is a cross sectional view schematically showing a structure of aphase shifting mask according to a third embodiment of the presentinvention.

FIG. 20 is a partial cross sectional view showing in an enlarged mannera region D₁ shown in FIG. 19.

FIGS. 21-28 are cross sectional views schematically showing in thisorder a manufacturing method of the phase shifting mask according to thethird embodiment of the present invention.

FIGS. 29 and 30 are cross sectional views for illustrating a case when aremaining defect is generated in the manufacturing method of the phaseshifting mask according to the third embodiment of the presentinvention.

FIG. 31A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the third embodiment of the presentinvention when used to a pattern in which the dense pattern and theisolated pattern are mingled.

FIG. 31B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 31A.

FIG. 32A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the third embodiment of the presentinvention when used to a pattern in which the dense pattern and theisolated pattern are mingled.

FIG. 32B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 32A.

FIG. 33 is a cross sectional view schematically showing a structure of aphase shifting mask according to a fourth embodiment of the presentinvention.

FIG. 34 is a partial cross sectional view showing in an enlarged mannera region D₂ shown in FIG. 33.

FIGS. 35-42 are cross sectional views schematically showing in thisorder a manufacturing method of the phase shifting mask according to thefourth embodiment of the present invention.

FIG. 43A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the fourth embodiment of thepresent invention when used to a pattern in which the dense pattern andthe isolated pattern are mingled.

FIG. 43B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 43A.

FIG. 44A is a cross sectional view schematically showing a structure ofthe phase shifting mask according to the fourth embodiment of thepresent invention when used to a pattern in which the dense pattern andthe isolated pattern are mingled.

FIG. 44B is a cross sectional view schematically showing a resistpattern formed by the phase shifting mask shown in FIG. 44A.

FIG. 45 is a schematic view of an exposure method using a conventionalphase shifting mask.

FIG. 46A is a cross sectional view of a mask when a conventionalphotomask is used.

FIG. 46B is a graph showing an electric field on the mask when thephotomask shown in FIG. 46A is used.

FIG. 46C is a graph showing light intensity on the wafer when thephotomask shown in FIG. 46A is used.

FIG. 47A is a cross sectional view of a mask when a phase shifting maskof the Levenson system is used.

FIG. 47B is a graph showing an electric field of the mask when the phaseshifting mask shown in FIG. 47A is used.

FIG. 47C is a graph showing light intensity on the wafer when the phaseshifting mask shown in FIG. 47A is used.

FIG. 48A is a cross sectional view of a mask when a phase shifting maskof the halftone system is used.

FIG. 48B is a graph showing an electric field on the mask when the phaseshifting mask shown in FIG. 48A is used.

FIG. 48C is a graph showing light intensity on the wafer when the phaseshifting mask shown in FIG. 48A is used.

FIG. 49 is a cross sectional view schematically showing a structure of aconventional phase shifting mask.

FIGS. 50-57 are cross sectional views schematically showing in thisorder a manufacturing method of the conventional phase shifting mask.

FIGS. 58-61 are cross sectional views schematically showing in thisorder a manufacturing method of a phase shifting mask disclosed in priorart documents.

FIGS. 62 and 63 are cross sectional views for illustrating adisadvantage in the manufacturing method of the phase shifting maskdisclosed in prior art documents.

FIGS. 64 and 65 are cross sectional views for illustrating an advantageof the manufacturing method of the conventional phase shifting mask.

FIG. 66 is a cross sectional view schematically showing a structure ofthe conventional phase shifting mask of the halftone system.

FIG. 67A is a cross sectional view showing the conventional phaseshifting mask of the Levenson system for illustrating a disadvantagewhen an isolated pattern is formed by using the conventional phaseshifting mask of the Levenson system.

FIG. 67B is a graph showing light intensity on the wafer when the phaseshifting mask shown in FIG. 47A is used.

FIG. 68A is a cross sectional view schematically showing theconventional phase shifting mask of the halftone system for illustratinga disadvantage when a dense pattern is formed by using the conventionalphase shifting mask of the halftone system .

FIG. 68B is a graph showing light intensity on the wafer when the phaseshifting mask shown in FIG. 68A is used.

FIG. 69 is a cross sectional view schematically showing the conventionalphase shifting mask for illustrating degradation of adherence of thelight shielding film.

FIGS. 70-73 are cross sectional views of the conventional phase shiftingmask for illustrating a disadvantage caused by remaining of a foreignobject.

FIG. 74 is a cross sectional view schematically showing the conventionalphase shifting mask for illustrating a disadvantage when a foreignobject is left on a stepped portion of the light shielding film.

FIGS. 75-77 are cross sectional views of the conventional phase shiftingmask for illustrating a disadvantage caused by a remaining defect.

FIG. 78 is a cross sectional view schematically showing the conventionalphase shifting mask for illustrating a disadvantage when a remainingdefect is partially generated.

FIG. 79 is a plan view schematically showing a structure of aninterconnection layer patterned by a phase shifting mask in which aremaining defect is partially generated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below inconjunction with the drawings.

Embodiment 1

Referring to FIG. 1, a phase shifting mask 20 according to a firstembodiment includes a quartz substrate 1 and a semi-light shielding film3. A trench is formed on the surface of quartz substrate 1. A region inwhich the trench is not formed serves as a first light transmittingportion 1a, while a region in which the trench is formed serves as asecond light transmitting portion 1b. These first and second lighttransmitting portions 1a and 1b are structured such that phases ofexposure light transmitted through respective portions are out of phaseby 180° with each other.

First light transmitting portion 1a includes a first transmitting region1L which is not covered by semi-light shielding film 3, and a firstattenuated transmitting region 1N₁ which is covered by semi-lightshielding film 3. Second light transmitting portion 1b includes a secondlight transmitting region 1M which is not covered by semi-lightshielding film 3, and a second attenuated transmitting region 1N₂ whichis covered by semi-light shielding film 3.

A sidewall of the trench is substantially perpendicular to a bottom wallthereof. A stepped portion is formed on the sidewall of the trench.Semi-light shielding film 3 is formed on the surface of quartz substrate1 so as to cover the stepped portion and to expose predetermined regionsof first and second light transmitting portions 1a and 1b.

Semi-light shielding film 3 has transmittance of at least 3% and notmore than 30%. The phases of exposure light after transmitting throughsemi-light shielding film 3 are converted by 0°, 360°, 360°×2, . . . . ,360°×n, . . . with respect to phases of exposure light beforetransmitting through the film. In other words, semi-light shielding film3 does not convert the phase of transmitted light substantially, andmaintains phases of exposure light before and after transmission throughsemi-light shielding film 3 substantially equal.

As a material of semi-light shielding film 3, chromium (Cr) film, forexample, is used. A specific thickness of the chromium film will beabout 200 Å when an i-line is used as exposure light, and about 150 Åwhen KrF excimer laser is used, providing 10% of exposure light istransmitted. Also it is sufficient to set the thickness of the chromiumfilm within a range of 100 Å-300 Å, considering the wavelength ofexposure light used and transmittance to be set.

A depth of the trench (or a height of the stepped portion) will be about4050 Å when the i-line is used as exposure light and about 2720 Å whenKrF excimer laser is used, in order to provide a phase differencebetween first and second light transmitting portions 1a and 1b.

Next, a manufacturing method of a phase shifting mask of this embodimentwill be described.

Referring to FIG. 2, after a quartz substrate having a thickness of 6.35mm is prepared, a trench is formed on the surface of quartz substrate 1by the step as in the conventional method. Quartz substrate 1, and firstand second light transmitting portions 1a and 1b are thus formed.

A chromium film having a thickness of 100 Å-300 Å is then formed on theentire surface so as to cover the trench. A resist pattern 9 having adesired shape is formed on the surface of chromium film 3. Using resistpattern 9 as a mask, chromium film 3 is subjected to anisotropicetching, thereby forming a semi-light shielding film 3 which forms thesidewall of the trench and exposes desired regions of first and secondlight transmitting portions 1a and 1b. After that, resist pattern 9 isremoved, whereby phase shifting mask 20 as shown in FIG. 1 is completed.

In the phase shifting mask of this embodiment, a high resolution can beobtained in both a dense pattern and an isolated pattern, for which adetailed description will be given below.

Referring to FIGS. 3A and 3B, adjacent transmitting regions, (regionsnot covered by semi-light shielding film 3) 1L and 1M are disposed inproximity to each other in the case of the dense pattern. Accordingly,beams of exposure light transmitted through each of adjacenttransmitting regions 1L and 1M are overlapped at an edge of an exposurepattern, as shown by a dash-dot line in the figure. Since theseoverlapping beams of exposure light are out of phase with each other,that is, the phases are reverse, they will be canceled with each other.Thus, light intensity of the exposure pattern necessarily includes aportion having a light intensity of zero at the edge portion (betweenadjacent exposure patterns) as shown by a solid line in the figure.Therefore, a sharp exposure pattern can be obtained at the edge portion,whereby a sufficient difference of light intensity can be provided toimprove the resolution.

It is noted that in a region k₁ indicated in the figure, light intensityof the exposure pattern is enhanced locally because light shielding film3 does not intercept the exposure light completely and transmits theexposure light to a certain extent.

As for the isolated pattern, referring to FIG. 4, adjacent transmittingregions are spaced apart by a considerable distance from each other.Accordingly, beams of exposure light transmitted through each ofadjacent transmitting regions 1M and 1M (or 1M and 1L, or 1L and 1L) arenot overlapped. In the meantime, in this embodiment, transmission ofsemi-light shielding film 3 is set at least 3% and not more than 30% soas to permit transmission of some of the exposure light. This means thatattenuated transmitting region (a region covered by light shielding film3) 1N also transmits the exposure light to a certain extent. Therefore,beams of exposure light transmitted through transmitting region 1M andattenuated transmitting 1N are overlapped at an edge portion of anexposure pattern. Since these overlapping beams of exposure light areout of phase with each other, they are canceled with each other. Thus,as can be seen by a solid line in the figure, light intensity of theexposure pattern necessarily includes a portion having a light intensityof zero at the edge portion thereof, whereby a sharp exposure patterncan be obtained at the edge portion to improve the resolution.

As described above, a good resolution can be obtained in both the densepattern and the isolated pattern in the phase shifting mask of thisembodiment. Therefore, even if the dense pattern and the isolatedpattern are mingled on the same mask, a high resolution can be obtainedupon application of this embodiment. In other words, a desired patternshape can easily be formed even though a circuit pattern becomes smallerand increases its complexity.

It should be noted that in the phase shifting mask of this embodiment,the intensity of exposure light should be adjusted such that aphotoresist is not photosensitized by the exposure light transmittedthrough semi-light shielding film 3, or a certain thickness of thephotoresist is left after development even when the photoresist isphotosensitized.

Referring to FIG. 5, in the case of exposure of a usual hole pattern,exposure light is directed to a mask with such amount of light as isabout 3-4 times the exposure amount by which no resist is left afterdevelopment (point R in the figure). If transmittance of semi-lightshielding film 3 has to be 30%, a thickness of the photoresist becomeszero due to the exposure light transmitted through semi-light shieldingfilm 3, or the photoresist is reduced and cannot be used as an etchingmask. In this respect, transmittance of semi-light shielding film 3 hasto be 30% or less.

In the meanwhile, if intensity of light transmitted through semi-lightshielding film 3 is too weak, an effect of obtaining a sharp exposurepattern by virtue of overlapping of beams of exposure light havingdifferent phases, as discussed with FIGS. 4A and 4B, cannot be provided.More particularly, if transmittance of semi-light shielding film 3 ismade less than 3%, light intensity of the exposure light transmittedthrough semi-light shielding film 3 becomes too small, prohibiting theabove effect. Therefore, semi-light shielding film 3 must havetransmittance of 3% or more.

Next, a specific structure of a phase shifting mask when the phaseshifting mask of this embodiment is applied to a pattern in which thedense pattern and the isolated pattern are mingled, and a mingledpattern formed by such a phase shifting mask will be described below.

Referring to FIG. 6A or FIG. 7A, a plurality of trenches are formedspaced apart a predetermined distance from each other on the surface ofquartz substrate 1. A region in which the trench is not formed serves asfirst light transmitting portion 1a, while a region in which the trenchis formed serves as second light transmitting portion 1b. Lightshielding film 3 is patterned to cover the sidewall of the trench and toexpose predetermined regions of first and second light transmittingportions 1a and 1b.

Referring to FIG. 6B or FIG. 7B, when the photoresist on a wafer 121 isexposed with light and developed with using the above-described phaseshifting mask, each resist pattern 121b, 121c is obtained. It is notedthat a negative photoresist is used in both cases. A region E₁₁ ofresist pattern 121b and a region E₁₂ of resist pattern 121c serve as thedense pattern. A region F₁₁ of resist pattern 121b is a so-calledaperture of the isolated pattern, while regions F₁₂ and F₁₃ of resistpattern 121c are so-called remaining portions of the isolated pattern.

Now, a relationship between a distance S₁ between adjacent transmittingregions 1L and 1M shown in FIG. 1 and a degree of overlapping ofexposure patterns by exposure light transmitted through both regions 1Land 1M will be discussed below.

FIG. 8 is a graph included in J. Miyazaki et al. "The effect of dutyratio of line and space in phase-shifting lithography" SPIE vol. 1927,55, pp. 677-685. This graph is concerned with the phase shifting mask ofthe Levenson system and the conventional mask, showing a relationshipbetween a dimension of the light shielding portion on the wafer (SpaceWidth) and a depth of focus (DOF).

In FIG. 8, the depth of focus is plotted on the ordinate, while thedistance between adjacent transmitting regions on the wafer (that is, adimension of the light shielding portion irradiated onto the wafer) isplotted on the abscissa. As can be seen from the figure, when thedimension of the light shielding portion of the mask is not more than0.7 μm on the wafer, the depth of focus of the phase shifting mask ofthe Levenson system is greater than that of the conventional mask.However, when the dimension is 0.7 μm or more, the phase shifting maskof the Levenson system and the conventional mask will have approximatelythe same depth of focus, diminishing a difference of the two.

Since the i-line which has a wavelength of 0.365 μm is used in thisexperiment, the dimension of 0.7 μm equals about two times (2λ) awavelength λ of the i-line. In other words, unless the dimension of thelight shielding portions irradiated onto the wafer is 2λ or less, thephase shifting effect of the Levenson system cannot be provided.

If it is desired that the dimension of the light shielding portionirradiated onto the wafer is converted to the dimension of the lightshielding portion on the phase shifting mask, a magnification of theprojection optical system shown in FIG. 45 should be considered.Referring to FIG. 45, a circuit pattern of a phase shifting mask 720 isusually reduced by a predetermined magnification by the projectionoptical system to be directed onto a photoresist 121a. Suppose that thecircuit pattern of phase shifting mask 720 is reduced by five times tobe directed onto photoresist 121a, the dimension of 5 μm on phaseshifting mask 720 would be the dimension of 1 μm on photoresist 121a.Therefore, assuming that the dimension of the light shielding portionirradiated onto a wafer 121 reduced by n times by the projection opticalsystem is 2λ, the dimension of the light shielding portion on the phaseshifting mask is 2λ×n.

Thus, in the phase shifting mask of the conventional Levenson system asshown in FIG. 49, the phase shifting effect cannot be provided unlessthe dimension of the light shielding portion on the phase shifting maskis about 2λ×n or less, for which the description will be given below.

Referring to FIGS. 9A and 9B, if a dimension S₁₀₀ of attenuatedtransmitting region 1N exceeds 2λ×n, beams of exposure light transmittedthrough adjacent transmitting regions 1L and 1M are not overlapped. Itis thus considered that the phase shifting effect cannot be provided andthe depth of focus becomes similar to that of the conventional mask, ifdimension S₁₀₀ of attenuated transmitting region 1N is greater than2λ×n.

On the contrary, referring to FIGS. 9A and 9C, beams of exposure lighttransmitted through adjacent transmitting regions 1L and 1M areoverlapped when the dimension of the attenuated transmitting region 1Nis smaller than 2λ×n. Therefore, the phase shifting effect can beprovided and the depth of focus is greater than that of the conventionalmask.

As described above, it is considered that whether beams of exposurelight transmitted through adjacent transmitting regions 1L and 1M areoverlapped depends on the dimension of attenuated transmitting region 1Nprovided between transmitting regions 1L and 1M being greater or smallerthan 2λ×n. If this concept is applied to this embodiment, the followingis the result.

Referring to FIGS. 3A and 3B, beams of light transmitted throughadjacent transmitting regions 1L and 1M are overlapped if the dimensionof attenuated transmitting region 1N is smaller than 2λ×n. Therefore,the phase shifting effect of the Levenson system can be obtained.

On the contrary, referring to FIGS. 4A and 4B, beams of exposure lighttransmitted through adjacent transmitting regions 1L and 1M are notoverlapped if the dimension of attenuated transmitting region 1N isgreater than 2λ×n. However, beams of exposure light transmitted throughtransmitted through transmitting region 1M and attenuated transmittingregion 1N are overlapped. Therefore, the phase shifting effect of thehalftone system can be obtained in this case.

As a result, in the phase shifting mask of this embodiment, the phaseshifting effect of the Levenson system can be obtained when dimensionS₁₀₀ of attenuated transmitting region 1N is made smaller than 2λ×n,while the effect of the halftone system can be obtained when dimensionS₁₀₀ is made larger than 2λ×n.

In this respect, when the circuit pattern in which the dense pattern andthe isolated pattern are mingled is formed as shown in FIGS. 7A and 7B,it is sufficient to set dimension S₃ of semi-light shielding film 3smaller than 2λ×n in the dense pattern region E₁₂, and set dimension S₂larger than 2λ×n in the isolated pattern region.

Now, an exposure method using the phase shifting mask of this embodimentwill be described.

Referring to FIG. 10, the exposure method of this embodiment is almostsimilar to a conventional exposure method except for a mask 20 and adiaphragm 114 used. Diaphragm 114 is different because coherency σ ofexposure light used when the phase shifting mask of this embodiment isused is different from that used when a conventional mask is used, forwhich description will be given below in more detail.

In the phase shifting masks of the Levenson system and the halftonesystem, a smaller coherency σ is preferred. Coherency σ represents adegree of interference of light, so that the smaller coherency σ, thegreater a degree of interference of light, while the greater coherencyα, the smaller a degree of interference of light. In other words, sincethe phase shifting mask is intended to improve a resolution byinterference of reverse-phased light, greater degrees of interference oflight are preferred. Thus, a smaller coherency σ is preferred when thephase shifting mask is used. In practice, however, coherency σ of thephase shifting mask is about 0.3 because of constraint such as reductionof the amount of exposure light.

On the contrary, smaller interference of light is preferred in theconventional mask, so that a larger coherency σ is preferred. Thus,coherency a is about 0.6 due to a structural constraint in theconventional mask.

When a circuit pattern including the dense pattern such as a memory cellregion 121M and the isolated pattern such as a peripheral circuit region121P as shown in FIG. 11 is formed, conventionally the phase shiftingmask and the conventional mask are used respectively for the densepattern and the isolated pattern. In other words, the phase shiftingmask and the conventional mask are mingled on the same mask.

In this case, if coherency σ of exposure light irradiated to the mask is0.3, then a resolution of the conventional mask is decreased, while ifcoherency σ is 0.6, then a resolution of the phase shifting mask isdecreased. Therefore, an intermediate coherency σ of about 0.4-0.5 hasbeen used. However, with this coherency σ (0.4-0.5), a so much highresolution cannot be obtained in both the phase shifting mask and theconventional mask. Therefore, it has been difficult to form a desiredpattern shape in the smaller circuit patterns.

In this embodiment, on the contrary, both dense pattern and isolatedpattern can be formed by virtue of the phase shifting effect. Thus, evenif the pattern includes both dense and isolated patterns, a highresolution can be obtained when coherency σ of the exposure lightdirected to the mask is set 0.3 which is suitable for the phase shiftingmask, whereby a desired pattern shape can easily be obtained.

Coherency σ is determined depending on a diameter of an aperture ofdiaphragm 114. Since coherency σ of this embodiment is different fromthat of the conventional example, a structure of diaphragm 114,especially its aperture diameter, is different. Diaphragm 114 of thisembodiment will be described below.

Referring to FIG. 12, diaphragm 114 includes a disk 114a having alight-shielding characteristic and an aperture 114b. Exposure light istransmitted through aperture 114b of diaphragm 114. Coherency a can bemade smaller by reducing diameter r of aperture 114b. The larger thediameter r, the greater coherency σ.

Embodiment 2

Referring to FIG. 13, a plurality of semi-light shielding films 203 areformed spaced apart a predetermined distance from each other on thesurface of a quartz substrate 201. A quartz layer 205 serving as ashifter is formed for every other surface of quartz substrate 201 whichis exposed from semi-light shielding film 203. A portion where shifter205 is formed serves as a first light transmitting portion 201a, while aportion where the shifter is not formed serves as a second lighttransmitting portion 201b. Phases of exposure light transmitting throughfirst and second light transmitting portions 201a and 201b are out ofphase with each other by about 180°.

First light transmitting portion 201a includes a first transmittingregion 201L where semi-light shielding film 203 is not formed and afirst attenuated transmitting region 201 N₁ where semi-light shieldingfilm 203 is formed. Second light transmitting portion 201b includes asecond transmitting region 201M where semi-light shielding film 203 isnot formed, and a second attenuated transmitting region 201N₂ wheresemi-light shielding film 203 is formed.

As semi-light shielding film 203, a chromium film, for example, is used.A thickness of the chromium film is about 200 Å when the i-line is usedas exposure light, and about 150 Å when KrF excimer laser is used,providing 10% of exposure light is transmitted.

Also, it is sufficient to set the chromium film to have a thicknesswithin the range of 100 Å-300Å, since the thickness changes according toa wavelength of exposure light used and transmittance to be set.

Now, a manufacturing method of this embodiment will be described. First,referring to FIG. 14, a chromium film having a thickness of, forexample, 200 Å is formed on the surface of quartz substrate 201. Bypatterning the chromium film by photolithography, semi-light shieldingfilm 203 having a desired shape is formed.

Referring to FIG. 15, a quartz layer 205a is formed on the entiresurface of quartz substrate 201 so as to cover semi-light shielding film203.

Referring to FIG. 16, a resist pattern 209 having a desired shape isformed on the surface of quartz layer 205a. Using resist pattern 209 asa mask, anisotropic etching is conducted to quartz layer 205. A shifter205 made of quartz is formed for every other surface of quartz substrate201 which is exposed from semi-light shielding film 203 by this etching.Thus, a region in which shifter 205 is formed serves as a first lighttransmitting portion 201a, while a region in which the shifter is notformed serve as a second light transmitting portion 201b. Resist pattern209 is then removed, whereby the phase shifting mask shown in FIG. 13 isobtained.

In the phase shifting mask of this embodiment, semi-light shielding film203 is formed in a part of first and second light transmitting portions,and has transmittance of at least 3% and not more than 30%. Thus, as inthe first embodiment, a high resolution can be obtained in both densedand isolated patterns, whereby a desired pattern shape can easily beformed even in a complex circuit pattern.

When the phase shifting mask of this embodiment is applied to a patternin which the dense pattern and the isolated pattern are mingled, a phaseshifting mask having a structure as shown in FIG. 17A and FIG. 18A, forexample, can be obtained.

Referring to FIGS. 17A and 18A, a plurality of semi-light shieldingfilms 203 are formed spaced apart by a desired distance from each otheron the surface of quartz substrate 201. Shifter 205 is formed on thesurface of quartz substrate 201 which is exposed from semi-lightshielding film 203 or on the surface of semi-light shielding film 203. Aregion in which shifter 205 is formed serves as first light transmittingportion 201a, while a region in which shifter 205 is not formed servesas second light transmitting portion 201b. Phases of exposure lighttransmitted through first and second light transmitting portions 201aand 201b are different from each other by 180°.

Referring to FIGS. 17B and 18B, resist patterns 121d and 121e are formedon wafer 121 by using the above-described phase shifting mask. RegionE₂₁ of resist pattern 121d and region E₂₂ of resist pattern 121erepresent the dense pattern region. Region F₂₁ of resist pattern 121drepresents the so-called aperture of the isolated pattern, while regionsF₂₂ and F₂₃ of resist pattern 121e represent the so-called remainingportions of the isolated pattern.

In the phase shifting mask of this embodiment, as in the firstembodiment, either the Levenson system or the halftone system can beselected to obtain the phase shifting effect by making dimension S₅ ofsemi-light shielding film 203 shown in FIG. 13 smaller or larger than2λ×n (n: magnification of the projection optical system).

More specifically, in FIGS. 17A and 17B, a dimension S₆ of semi-lightshielding film 203 is set less than 2λ×n in the dense pattern regionE₂₁, while a dimension S₇ is set more than 2λ×n in the isolated patternregion. Accordingly, the phase shifting effect of the Levenson systemand the halftone system can be provided respectively in the densepattern region E₂₁ and the isolated pattern region E₂₂. Therefore, ahigh resolution can be provided in both densed and isolated patterns.

Also, in the phase shifting mask of this embodiment, as in the firstembodiment, both densed and isolated patterns can be formed by virtue ofthe phase shifting effect. Thus, a high resolution can be obtained evenin the mixed pattern by setting coherency σ of exposure light to be avalue (0.3) which is appropriate to the phase shifting mask.

Embodiment 3

Referring to FIGS. 19 and 20, a phase shifting mask 320 according to athird embodiment includes a quartz substrate 301 and a light shieldingfilm 303. A trench is formed on the surface of quartz substrate 301. Aregion in which the trench is not formed serves as a first lighttransmitting portion 301a, and a region in which the trench is formedserves as a second light transmitting portion 301b. Beams of exposurelight transmitted through first and second light transmitting portions301a and 301b, respectively, are 180° out of phase with each other.

The surface of first light transmitting portion 301a is formed at aposition having a first height from the bottom surface of substrate 301.In the meanwhile, the surface of second light transmitting portion 301bis formed at a position having a second height which is lower than thefirst height from the bottom surface of quartz substrate 301. Thus,there is provided a stepped portion between first and second lighttransmitting portions 301a and 301b. This stepped portion has a radiusof curvature R₃₂ which is substantially equal to a height R₃₁ of thestepped portion.

A light shielding film 303 is formed on the surface of quartz substrate301 so as to cover the stepped portion and to expose predeterminedregions of first and second light transmitting portions 301a and 301b.

Now, a manufacturing method of the phase shifting mask of thisembodiment will be described.

Referring to FIG. 21, the necessary cleaning and preprocessing arecarried out to quartz substrate 301 which is made of synthetic quartzformed into a required shape and flatness. Then, a doped amorphoussilicon film 305a having a thickness of 300 Å-1000 Å is formed on quartzsubstrate 301 by an ordinary DC electric discharge sputtering apparatus.An electron beam resist film 307 having a thickness of 3000 Å or less isformed on the entire surface of doped amorphous silicon film 305a by anordinary spin coater. Onto this electron beam resist film 307a, ashifter pattern of the phase shifting mask of the Levenson type is drawnby an electron beam lithography apparatus.

After lithography, referring to FIG. 22, a resist pattern 307 is formedthrough development by an ordinary development apparatus. Using resistpattern 307 as a mask, doped amorphous silicon film 305a is selectivelyetched over quartz substrate 301 with plasma of a mixed gas having a CF₄:O₂ ratio of 95:5 by the plasma etching apparatus.

An etching selectivity at this time is about 60. Thus, an etched amountof the quartz substrate when doped amorphous silicon film 305a isoveretched by 100% is 5-16 Å, that is, about 10 Å. An influence,therefore, of the etching upon a phase angle of the shifter isnegligible. Then, electron beam resist pattern 307 is removed by ashingusing the ordinary oxygen plasma. After that, cleaning is carried out byan alkali cleaning solution.

Referring to FIG. 23, a shifter pattern is thus formed in dopedamorphous silicon film 305. A defect test of the shifter pattern isconducted by an ordinary optical pattern defect inspection apparatus.Commonly in such a mask process, it is unlikely that clear defects aregenerated, so that only opaque defects are generated and detected. Sinceopaque defects can be repaired by the above-described ordinary baserrepairing apparatus, no defects will be left.

Referring to FIG. 24, a surface processing is carried out by oxygenplasma in order to improve wettability (that is, to facilitate wetting)during wet etching. Wet etching is then carried out to quartz substrate301 for a desired amount (about 4100 Å in the mask for i-line exposure)by using a solution having a mixture ratio of ammonium fluoride andhydrofluoric acid of 50:1 plus active agent. The shifter pattern isformed in quartz substrate 301 by this wet etching.

During wet etching, doped amorphous silicon film 305 serves as anetching mask material against quartz substrate 301. Since there issufficient adherence between doped amorphous silicon film 305 and quartzsubstrate 301, an etchant is unlikely to enter an interface betweendoped amorphous silicon film 305 and quartz substrate 301.

If the etchant is likely to enter the interface, quartz substrate 301 isremoved by etching along the interface, as shown by a dotted line in thefigure, whereby peeling-off of amorphous silicon film 305 isfacilitated. However, as described above, the etchant is unlikely toenter the interface thanks to a good adherence, doped amorphous siliconfilm 305 will not be peeled off.

In the meanwhile, the etching rate of quartz substrate 301 during wetetching is sufficiently small. Therefore, controllability of a phaseshifting angle determined by the etching amount can be within ±5°. Dopedamorphous silicon film 305 is then removed by plasma of the mixed gashaving a CF₄ :O₂ ratio of 95:5. At this time, the etching selectivitybetween doped amorphous silicon film 305 and quartz substrate 301 isabout 60. Thus, an etched amount of quartz substrate 301 when dopedamorphous silicon film 305 is overetched by 100% is 5-16 Å, that is,about 10 Å. Therefore, the influence of etching upon phase shiftingangles of the shifter is negligible.

Referring to FIG. 25, first and second light transmitting portions 301aand 301b are thus formed in quartz substrate 301. Then, washing iscarried out by the alkali cleaning solution.

Referring to FIG. 26, a chromium film 303a having a thickness of about1000 Å which will serve as a light shielding material of the phaseshifting mask is formed on the entire surface by the ordinary DCdischarge sputtering apparatus. An electron beam resist film 309a havinga thickness of about 3000 Å is formed on the entire surface of chromiumfilm 303a by the ordinary spin coater. Then, a desired shape is drawn onelectron beam resist film 309a by the electron beam lithographyapparatus.

After lithography, referring to FIG. 27, an electron beam resist pattern309 is formed through development by the ordinary development apparatus.

During lithography using the electron beam, accuracy required inalignment with the underlying shifter pattern is sufficient to the levelof that of the ordinary electron beam lithography apparatus. In otherwords, the accuracy of the ordinary electron beam lithography apparatusis about 0.1 μm which is sufficiently smaller than that of underlyingpattern shape, so that the accuracy of alignment with the underlyingpattern is sufficient to the level of that of the electron beamlithography apparatus.

After that, electron beam resist pattern 309 is removed. Also, usualdefect inspection and repairs are carried out, whereby a desired phaseshifting mask 320 of the Levenson system as shown in FIG. 28 iscompleted.

As described with reference to FIGS. 19 and 20, in the phase shiftingmask of this embodiment, the stepped portion between first and secondlight transmitting portion 301a and 301b has a radius of curvature R₃₂which is substantially equal to height R₃₁ of the stepped portion. Thus,a slope of the stepped portion of this embodiment is gentler than thatof the conventional phase shifting mask 720 shown in FIG. 49. Thus,adherence of light shielding film 303 formed to cover the steppedportion to quartz substrate 301 is improved compared to conventionalphase shifting mask 720.

Also, the gentle slope of the stepped portion of quartz substrate 301 inphase shifting mask 320 of this embodiment prevents at the bottom of thestepped portion trapping of a foreign object which is generated duringremoval of doped amorphous silicon film 305 in the steps shown in FIGS.24 and 25. Thus, the foreign object is prevented from being left at thebottom of the stepped portion.

In the manufacturing method of this embodiment, isotropic etching iscarried out to quartz substrate 301 in the steps shown in FIGS. 23 and24. Thus, even if a remaining defect 305b of doped amorphous siliconfilm 305 is left as shown in FIG. 29, the underlying layer of remainingdefect 305b is also removed by etching.

More particularly, with reference to FIG. 30, spreading of the etchantis good in isotropic etching. Accordingly, the etchant spreads undermask 305 or remaining defect 305b. This causes regions of quartzsubstrate 301 located under mask 305 and remaining defect 305b to beremoved by etching. When thus removed by etching, remaining defect 305bfalls from quartz substrate 301 with the underlying layer thereof beinglost. This eliminates the possibility that a region to be removed is notremoved because of remaining defect 305b. When the resist film on thewafer is exposed with light by using the thus manufactured phaseshifting mask, a favorable resolution can be obtained so that nounsatisfactory pattern shape is provided.

In the phase shifting mask of this embodiment, the semi-light shieldingfilm used in the first and second embodiments can be used instead oflight shielding film 303. A chromium film is used, for example, as thesemi-light shielding film. A thickness of chromium is about 200 Å whenthe i-line is used as exposure light, and about 150 Å when KrF excimerlaser is used, providing 10% of exposure light is transmitted. Also, thethickness of the chromium film changes depending on a wavelength ofexposure light used and transmittance to be set, so that it issufficient to set the thickness within the range of 10 Å-300 Å.

If the semi-light shielding film is used in this embodiment instead ofthe light shielding film, then the same effect as can be obtained in thefirst and second embodiments can be provided.

More particularly, referring to FIG. 19, semi-light shielding film 303is formed in a part of first and second light transmitting portions 301aand 301b, and has transmittance of at least 3% and not more than 30%. Asin the first and second embodiments, therefore, a high resolution can beobtained in both densed and isolated patterns. Therefore, a desiredpattern shape can easily be formed even in a complex circuit pattern.

One specific example of the phase shifting mask when the phase shiftingmask of this embodiment is applied to the mix pattern in which thedensed and isolated patterns are mingled is as shown in FIGS. 31A and32A.

Referring to FIGS. 31A and 32A, a plurality of trenches are formedspaced apart by a desired distance from each other on the surface ofquartz substrate 301. The sidewall of the trench has substantially thesame radius of curvature as a depth of the trench. A region in whichtrench is not formed serves as first light transmitting portion 301a,and a region in which a trench is formed serves as second lighttransmitting portion 301b.

Beams of exposure light transmitted through first and second lighttransmitting portions 301a and 301b are 180° out of phase with eachother. Semi-light shielding film 303 is formed to cover the sidewall ofthe trench and to expose predetermined regions of first and second lighttransmitting portions 301a and 301b.

Also, in the phase shifting mask of this embodiment, either the Levensonsystem or the halftone system can be selected to obtain the phaseshifting effect by making a dimension S₁₀ of semi-light shielding film303 smaller or larger than 2λ×n (n:magnification of the projectionoptical system) as shown in FIG. 19.

More specifically, a dimension S₁₁ of semi-light shielding film 303 ismade smaller than 2λ×n in the dense pattern region E₃₁, while adimension S₁₂ of semi-light shielding film 303 is made larger than 2λ×n,as shown in FIGS. 31A and 31B. Therefore, the phase shifting effect ofthe Levenson system and the halftone system can be obtained respectivelyin the dense pattern region E₃₁ and the isolated pattern region. Thus, ahigh resolution can be obtained in both isolated and densed patterns.

As in the first embodiment, both dense pattern and isolated pattern canbe formed by using the phase shifting effect in the phase shifting maskof this embodiment. Therefore, a high resolution can be obtained bysetting coherency σ of exposure light to be a value (0.3) which isappropriate to the phase shifting mask.

Embodiment 4

Referring to FIGS. 33 and 34, a phase shifting mask 420 according to afourth embodiment includes a quartz substrate 411, an etching stopperfilm 413, a silicon oxide film (SOG:Spin On Glass) 415, and a lightshielding film 403. Etching stopper film 413 and silicon oxide film 415are stacked successively on the surface of quartz substrate 411. Quartzsubstrate 411, etching stopper film 413, and silicon oxide film 415constructs a substrate 401 which transmits exposure light.

Etching stopper film 413 is made by, for example, SnO, Al₂ O₃, or thelike, and has a thickness of 100 Å-1000 Å. Silicon oxide film 415 has athickness of about 4000 Å. A trench is formed in silicon oxide film 415such that part of the surface of etching stopper film 413 is exposed atthe bottom of the trench. A region in which the trench is not formedserves as a first light transmitting portion 401a, and a region in whichthe trench is formed serves as a second light transmitting portion 401b.

A difference in height between the top surface of etching stopper film413 and the top surface of silicon oxide film 415 is equivalent to athickness of silicon oxide film 415. A stepped portion is thus formed bytop surfaces of etching stopper film 413 and silicon oxide film 415.More particularly, a stepped portion is formed between surfaces of firstand second light transmitting portions 401a and 401b. The steppedportion has substantially the same radius of curvature R₄₂ as a heightR₄₁ of the stepped portion.

Light shielding film 403 made of chromium (Cr) is formed to cover thestepped portion which is formed by the sidewall of the trench, and toexpose desired regions of first and second light transmitting portions401a and 401b.

Now, a manufacturing method of the phase shifting mask according to thefourth embodiment of the present invention will be described below.

First, referring to FIG. 35, an etching stopper film 413, a siliconoxide film 415, and a doped amorphous silicon film 405a are formedstacked successively on the entire surface of quartz substrate 411 tohave thicknesses of 100 Å-1000 Å, not more than 4000 Å, and 300 Å-1000Å, respectively. An electron beam resist film 407a having a thickness of3000 Å is formed on the entire surface of doped amorphous silicon film405a by the ordinary spin coater. A shifter pattern of the phaseshifting mask of the Levenson system is drawn on electron beam resistfilm 407a by the electron beam lithography apparatus.

After lithography, referring to FIG. 36, a resist pattern 407 is formedthrough development by the ordinary development apparatus. Using resistpattern 407 as a mask, doped amorphous silicon film 405 is selectivelyetched over silicon oxide film 415 by plasma of a mixed gas having a CF₄:O₂ ratio of 95:5 by the isotropic plasma etching apparatus.

At this time, an etching selectivity of doped amorphous silicon film 405over silicon oxide film 415 is about 60. Thus, an etched amount ofsilicon oxide film 415 when doped amorphous silicon film is overetchedby 100% is 5 Å-16 Å, that is, about 10 Å. This indicates that aninfluence of the etching upon a phase angle of the shifter isnegligible. Electron beam resist pattern 407 is then removed by ashingusing the ordinary oxygen plasma, and cleaning is carried out with thealkali cleaning solution.

Referring to FIG. 37, a shifter pattern is thus formed in dopedamorphous silicon film 405. Defect inspection of the shifter pattern isconducted by the ordinary optical defect inspection apparatus. As inmost masking processes, it is unlikely that clear defects are generated,so that only opaque defects are generated and detected. Since opaquedefects can be repaired by the ordinary laser repairing apparatus, anydefects can be eliminated by such a repairing.

After that, the surface processing is carried out by the oxygen plasmain order to improve wettability (that is, to facilitate wetting) duringwet etching. Then, wet etching is carried out until the surface ofetching stopper film 413 is exposed by using a solution having a mixtureratio of ammonium fluoride to hydrofluoric acid of 50:1 plus a surfaceactive agent, thus forming the shifter pattern in substrate 401.

At this time, there is provided a sufficient adherence between dopedamorphous silicon film 405 serving as an etching mask and silicon oxidefilm 415. Therefore, there is no possibility that doped amorphoussilicon 405 is peeled off. At the same time, etching stopper film 413serves as an etching stopper during wet etching of silicon oxide film415. Therefore, controllability of a phase shifting angle can be verymuch improved.

Referring to FIG. 38, after the above wet etching, a part of siliconoxide film 415 is removed and a part of the surface of etching stopperfilm 413 is exposed. Then, doped amorphous silicon film 405 is removedby plasma of a mixed gas having a CF₄ :O₂ ratio of 95:5 by the isotropicplasma etching apparatus.

Referring to FIG. 39, substrate 401 (which consists of quartz substrate411, etching stopper film 413 and silicon oxide film 415) includingfirst and second light transmitting portions 401a and 401b can thus beprovided. Then, cleaning is carried out by using the alkali cleaningsolution.

After cleaning, referring to FIG. 40, a chromium film 403a which willserve as a light shielding material having a thickness of about 1000 Åis formed to cover the entire surface by the ordinary DC dischargesputtering apparatus. An electron beam resist film 409a having athickness of about 3000 Å is formed on the entire surface of chromiumfilm 403a by the ordinary spin coater. A desired pattern shape is drawnon electron beam resist film 409a by the electron beam lithographyapparatus.

After lithography, referring to FIG. 41, a resist pattern 409 is formedthrough development by the ordinary development apparatus.

During lithography by the electron beam, an accuracy required inalignment with the underlying shifter pattern is sufficient to the levelof that of the ordinary electron beam lithography apparatus. In otherwords, an accuracy of the ordinary electron beam lithography apparatusis about 0.1 μm which is sufficiently smaller than the underlyingpattern shape, so that the level of alignment accuracy of the electronbeam lithography apparatus is sufficient.

By using electron beam resist pattern 409 as a mask, chromium film 403ais selectively etched to form a light shielding pattern 403 made ofchromium film. Electron beam resist pattern 409 is then removed by theordinary ashing using oxygen plasma. After that, usual defect inspectionand repairs are carried out, thus completing a desired phase shiftingmask 420 of the Levenson system shown in FIG. 42.

In the phase shifting mask of this embodiment, the stepped portionformed between surfaces of first and second light transmitting portions401a and 401b has substantially the same radius of curvature R₄₂ as aheight R₄₁ of the stepped portion. In other words, a sidewall of thestepped portion of this embodiment is gentler than a sidewall of thestepped portion in conventional phase shifting mask 720 in FIG. 49.Therefore, even if light shielding film 403 is formed on the steppedportion of this embodiment, a good adherence of light shielding film 403to silicon oxide film 415 can be provided.

Also, the gentle slope of stepped portion prevents at the bottom ofstepped portion trapping of a foreign object which is generated duringremoval of doped amorphous silicon film 405 in the steps shown in FIGS.38 and 39.

Also, in the manufacturing method of the phase shifting mask of thisembodiment, isotropic etching is carried out to silicon oxide film 115in the steps shown in FIGS. 37 and 38. During this isotropic etching, anetchant spreads under mask 405 and remaining defects, (not shown).Therefore, as described with reference to FIGS. 29 and 30 in the thirdembodiment, even if remaining defects are generated in this embodiment,there is no possibility that a region to be removed by etching is left.When the resist film on the wafer is exposed with light by using thethus manufactured phase shifting mask, a good resolution can be obtainedand no unsatisfactory pattern shape is formed.

In the meantime, in this embodiment, substrate 401 is not formed by asingle layer as in the third embodiment. More particularly, substrate401 consists of three layers of quartz substrate, 411, etching stopperfilm 413 and silicon oxide film 415. Etching stopper film 413 serves asan etching stopper during etching of silicon oxide film 415. It isunlikely that etching stopper film 413 is etched during etching ofsilicon oxide film 415. Therefore, a distance d₁ between surfaces offirst and second light transmitting portions 401a and 401b can easily becontrolled, as shown in FIG. 39.

More particularly, since etching stopper film 413 is provided undersilicon oxide film 415, the distance between surfaces of first andsecond light transmitting portions 401a and 401b can easily be adjustedto a desired distance d₁ without strictly controlling an etching amountof silicon oxide film 415. In the meanwhile, a difference in phaseshifting angles of first and second light transmitting portions 401a and401b can be determined according to this distance d₁. Therefore, thethree-layered structure of substrate 401 facilitates control of thedifference in phase shifting angles of first and second lighttransmitting portions 401a and 401b, whereby controllability of phaseshifting angles can be very much improved.

It is noted that the semi-light shielding film used in the first andsecond embodiments can be used instead of light shielding film 403 inthe phase shifting mask of this embodiment. A chromium film, forexample, is used for the semi-light shielding film. The chromium filmhas a thickness of about 200 Å when the i-line is used as exposurelight, and about 150 Å when KrF excimer laser is used, providing 10% ofexposure light is transmitted.

Also, it is sufficient to set the thickness of the chromium film withinthe range of 100 Å-300 Å, because the thickness changes depending on awavelength of exposure light used and transmittance to be set.

One specific example of the phase shifting mask is as shown in FIGS. 43Aand 44A, when the phase shifting mask of this embodiment using thesemi-light shielding film is applied to the mixed pattern of dense andisolated patterns.

Referring to FIGS. 43A and 44A, etching stopper film 413 is formed onthe entire surface of quartz substrate 411. Silicon oxide film 415 isformed on a predetermined region of etching stopper film 413. A trenchreaching etching stopper film 413 is formed in silicon oxide film 415.The sidewall of the trench has substantially the same radius ofcurvature as a thickness of silicon oxide film 415.

A portion in which silicon oxide film 415 is formed serves as firstlight transmitting portion 401a, and a portion in which silicon oxidefilm 415 is not formed serves as second light transmitting portion 401b.Semi-light shielding film 403 is formed to cover the curved sidewallhaving a predetermined curvature of silicon oxide film 415 and to exposepredetermined regions of first and second light transmitting portions401a and 401b.

Resist patterns formed by using the phase shifting masks shown in FIGS.43A and 44A are illustrated respectively in FIGS. 43B and 44B. A regionE₄₁ of resist pattern 121i and a region E₄₂ of resist pattern 121hrepresent the dense pattern region, while a region F₄₁ of resist pattern121i represents the so-called aperture of the isolated pattern. Also,regions F₄₂ and F4₃ of resist pattern 121h represent the so-calledremaining portions of the isolated pattern.

In the phase shifting mask of this embodiment using the semi-lightshielding film, either the Levenson system or the halftone system can beselected to obtain the phase shifting effect by making a dimension S₁.of semi-light shielding film 403 shown in FIG. 33 smaller or larger than2λ×n.

More specifically, when a dimension S₁₆ of semi-light shielding film 413in the dense pattern region E₄₁ is made smaller than 2λ×n, and adimension S₁₇ of semi-light shielding film 403 in isolated patternregion is made larger than 2λ×n, whereby the phase shifting effect ofthe Levenson system and the halftone system can be provided respectivelyin the dense pattern region E₄₁ and the isolated pattern region.Therefore, a high resolution can be obtained in both isolated and densepatterns.

Also in the phase shifting mask of this embodiment using the semi-lightshielding film, both the dense pattern and the isolated pattern can beformed by using the phase shifting effect. Therefore, it is possible toset coherency σ of exposure light to be a value (0.3) which isappropriate to the phase shifting mask, thus providing a high resolutionin both dense and isolated patterns.

It is noted that chromium has been used for the semi-light shieldingfilm 3, 203, 303, 403, in first through fourth embodiments, however, itis not limited thereto and any material can be applied so long as onlytransmittance of light can be controlled and a phase is not changedsubstantially.

This embodiment has been described when substrate 401 is formed by thethree-layered structure of quartz substrate 411, etching stopper film413, and silicon oxide film 415, however, the structure is not limitedthereto. More specifically, any structure can be applied so long as atleast two layers having different under etching characteristics arestacked, and the stacked structure may include three layers or more.

Also, this embodiment has been described in the case when etchingstopper film 413 is made of SnO or Al₂ O₃, however, materials used arenot limited thereto. More specifically, any material can be used so longas the material has transmittance of 90% or more and resists wet etchingof the silicon oxide film.

Further, silicon oxide film 415 can be made of other materials havingtransmittance of at least 90%. Also, it has been described that thesilicon oxide film has the thickness of 4000 Å, however, the thicknesschanges depending on the material. More specifically, a thickness d canassume any value as long as it satisfies the following expression:##EQU1## (where λ₀ : wavelength of exposure light, n:index of refractionof shifter material)

In first through fourth embodiments, quartz substrate 1, 201, 301, 411is not necessarily made of quartz, and it can be made of any materialhaving transmittance of at least 90%.

In third and fourth embodiments, doped amorphous silicon film 305, 405can be made of other materials. More specifically, any material can beused so long as it has conductivity and a superior resistance againsthydrofluoric acid such as MoSi₂. The reason why conductivity isimportant as a characteristic of such a material is that the resist filmapplied on the surface of such a film is exposed with light by theelectron beam lithography method. In other words, if the exposure iscarried out by the method other than the electron beam lithographymethod, conductivity is not required as a characteristic of such a film.

Light shielding film 303, 403 made of chromium (Cr) has been describedin the above, however, the film is not limited thereto. In other words,the film may be such a multi-layered film as is made of Cro/Cr/CrO.Characteristics required for light shielding film 303, 403 include notransmission of exposure light, a high resistance against chemicals(during cleaning), and adherence. Any material which satisfies thesecharacteristics can be used as light shielding film 303, 403.

Further, the dimensions and the materials described in first throughfourth embodiments are not limited thereto and can be selectedarbitrarily.

In a phase shifting mask according to one aspect of the presentinvention, a semi-light shielding film is located between first andsecond light transmitting portions, and formed at a region of thesefirst and second light transmitting portions. The semi-light shieldingfilm has transmittance of at least 3% and not more than 30%, whereby ahigh resolution can be obtained in a dense pattern and an isolatedpattern. Thus, a desired pattern shape can easily be obtained.

In a phase shifting mask according to one preferred aspect of thepresent invention, a sidewall of a stepped portion of a substrate is ina shape having substantially the same radius of curvature as a height ofthe stepped portion. Accordingly, when the semi-light shielding film isformed on the stepped portion, adherence of the semi-light shieldingfilm to the substrate is improved. Also, since a slope of the steppedportion is gentle, trapping of a foreign object which is generatedduring cleaning at the bottom of stepped portion can be prevented.Thus,a desired pattern shape can easily be obtained.

In a phase shifting mask according to another preferred aspect of thepresent invention, the substrate includes first and second films made ofmaterials having different under etching characteristics. Therefore,controllability of a phase shifting angle between first and second lighttransmitting portions can be very much improved. Thus, a desired patternshape can easily be obtained.

In a phase shifting mask according to another aspect of the presentinvention, a sidewall of a stepped portion of a substrate is in a shapehaving substantially the same radius of curvature as a height of thestepped portion. Therefore, when a light shielding film is formed on thestepped portion, adherence of the light shielding film to the substrateis improved.

Also, since a slope of the stepped portion is gentle, trapping of aforeign object which is generated during cleaning at the bottom at thestepped portion can be prevented.

In a phase shifting mask according to one preferred aspect of thepresent invention, the substrate includes first and second films made ofmaterials having different under etching characteristics. Therefore,controllability of a phase shifting angle between first and second lighttransmitting portions is very much improved. Thus, a desired patternshape can easily be obtained.

In an exposure method using the phase shifting mask according to oneaspect of the present invention, the phase shifting mask allowing a highresolution in both dense and isolated patterns is used. Therefore, adesired pattern shape can easily be obtained even in a circuit patternin which the dense and isolated patterns are mingled.

Also, since both the dense pattern and the isolated pattern can beformed by the phase shifting effect, coherency σ of exposure light canbe set to be an appropriate value. Thus, a desired pattern shape caneasily be obtained.

In an exposure method using the phase shifting mask according to anotheraspect of the present invention, the phase shifting mask which has agood adherence of the light shielding film to the substrate and in whicha foreign object is unlikely to be trapped is used. Therefore, anunsatisfactory pattern shape due to defects is not likely to generate,so that a desired pattern shape can easily be obtained.

In a manufacturing method of the phase shifting mask according to oneaspect of the present invention, a high resolution can be obtained inboth dense and isolated patterns. This permits manufacturing of thephase shifting mask in which a desired pattern shape can easily beobtained.

In a manufacturing method of the phase shifting mask according toanother aspect of the present invention, a main surface of the substrateis isotropically etched. A slope of the stepped portion formed by thisetching is gentler than that when anisotropic etching is carried out.Thus, when the light shielding film is formed on the stepped portion,adherence of the light shielding film to the substrate is improved.

The gentle slope of the stepped portion prevents trapping a foreignobject which is generated during cleaning at the bottom of steppedportion.

Further, an etchant spreads under a mask during isotropic etching. Evenif a remaining defect is left on the substrate, the etchant spreadsunder that remaining defect during etching, thus removing that portionof the substrate. Accordingly, the remaining defect is fallen offbecause the underlying layer is lost. Therefore, a region to be removedby etching on the substrate will not be left by isotropic etching evenwhen there is the remaining defect. Thus, degradation of resolution andformation of unsatisfactory pattern shape can be prevented.

In a manufacturing method of the phase shifting mask according to apreferred aspect of the present invention, the substrate includes firstand second films made of materials having different under etchingcharacteristics. Thus, controllability of a phase shifting angle betweenfirst and second light transmitting portions is very much improved, sothat the phase shifting mask which facilitates formation of a desiredpattern shape can be manufactured.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An exposure method, comprising the stepsof:emitting exposure light from a light source; directing said exposurelight to a phase shifting mask; and projecting the exposure lighttransmitted through said phase shifting mask onto a photoresist placedon a film to be etched for photosensitizing said photoresist,whereinsaid phase shifting mask includesa substrate including a firstlight transmitting portion which transmits said exposure light and asecond light transmitting portion which is adjacent to said first lighttransmitting portion and transmits exposure light to have a differentphase from that of the exposure light transmitted through said firstlight transmitting portion, and a semi-light shielding film which islocated at an interface between said adjacent first and second lighttransmitting portions and formed in a part of said first and secondlight transmitting portions, said first light transmitting portionincludes a first transmitting region and a first attenuated transmittingregion in which said semi-light shielding film is formed, lightintensity of exposure light transmitted through said first transmittingregion is greater than that of the exposure light transmitted throughsaid first attenuated transmitting region, said second lighttransmitting portion includes a second transmitting region and a secondattenuated transmitting region in which said semi-light shielding filmis formed, and light intensity of exposure light transmitted throughsaid second transmitting region is greater than that of the exposurelight transmitted through said second attenuated transmitting region. 2.An exposure method, comprising the steps of:emitting exposure light froma light source; directing said exposure light to a phase shifting mask;and projecting the exposure light transmitted through said phaseshifting mask onto a photoresist placed on a film to be etched forphotosensitizing said photoresist, whereinsaid phase shifting maskincludesa substrate including a first light transmitting portion whichtransmits said exposure light and a second light transmitting portionwhich transmits exposure light to have a different phase from that ofexposure light transmitted through said first light transmittingportion, surfaces of said first and second light transmitting portionsforming a stepped portion having a predetermined height, and a lightshielding film which covers the stepped portion of said substrate andexposes a predetermined region of said first and second lighttransmitting portions, and wherein a sidewall of the stepped portion ofsaid substrate has a round shape.